The present invention relates generally to a combustion detecting apparatus for an internal combustion engine. More particularly, the invention is concerned with a combustion detecting apparatus which is capable of detecting the state of combustion (normal combustion or abnormal combustion such as misfiring) by sensing an ion current generated by combustion of a mixture with enhanced reliability.
For a better understanding of the invention, a known combustion detecting apparatus for an engine will first be described by reference to FIG. 5 which is a circuit diagram showing the circuit arrangement of the apparatus. Referring to this figure, a power transistor 1, which is turned on and off in response to application of an ignition signal, has a collector connected to a primary winding of an ignition coil 2 which has a secondary winding connected to a movable contact of a distributor 3. A plurality of spark plugs, generally denoted by reference numeral 4, are connected to fixed or stationary contacts of the distributor 3, respectively. Further, a plurality of ion current sensing diodes 5 are connected to the corresponding spark plugs 4, respectively, and have respective output terminals connected together to an ion current detecting circuit 6 for detecting an ion current. The ion current detecting circuit 6 includes a comparator 7 for comparing an ion current detection signal with a reference value, and generating an output signal at an output terminal 8 thereof.
Operation of the combustion detecting apparatus implemented in the above structure will next be described by reference to a timing chart shown in FIG. 6. When an ignition pulse signal S101 having a waveform, as illustrated at (a) in FIG. 6, is applied to a base of the power transistor 1, the transistor is turned on. When the power transistor 1 is turned off at a falling edge of the ignition pulse signal, a high voltage signal S102 having a waveform illustrated at (b) in FIG. 6 is induced across the secondary winding of the ignition coil 2. The high voltage signal S102 thus generated is supplied to the spark plugs 4 of the individual engine cylinders through the distributor 3 to bring about combustion of an air/fuel mixture in the respective cylinders. As a result of this combustion, an ion current or flow is generated. Of the ion flow, only the positive ions or cations are detected by the diode 5, whereby an ion current sensing signal S103 having such a waveform as illustrated at (c) in FIG. 6 is generated and then compared with a reference value by the comparator 7. As a result of this, an ion current detection output signal S104 of a waveform shown at (d) in FIG. 6 is obtained at the output terminal 8. When the ion current detection output signal S104 is of a high level, decision is made that normal combustion took place. On the contrary, if the signal S104 is of a low level, occurrence of misfiring is determined.
The known combustion detecting apparatus for the internal combustion engine suffers from a disadvantage that reliability in the ion current detection and hence in decision as to the occurrence of combustion or misfiring is poor, involving erroneous decision. This can be explained by the fact that the detection level for comparison with the predetermined level becomes lowered particularly when the engine is operated at a high speed or under a heavy load, because the ion current detection is based on detection of the positive ions or cations produced upon combustion of the air/fuel mixture.
Further, the combustion detection apparatus known heretofore encounters such difficulty that when the apparatus is installed on a motor vehicle, noise originating in various noise sources in the vehicle will be superposed on the ion current detection output signal to thereby give rise to a high-level detection signal, erroneously indicating the normal combustion, even when misfiring took place, to a further disadvantage.